WASHINGTON — Sperm whales routinely dive more than two miles (3 kilometers) below the ocean surface to hunt for giant squid, but a study shows the huge mammals suffer a chronic loss of bone tissue from the bends, a painful condition well-known to human divers.

It has long been believed that sperm whales and other deep-diving mammals are immune from decompression illness, or the bends, which human divers encounter when they surface too rapidly and force nitrogen bubbles into their blood and tissues. Sperm whales have been known to dive as deeply as 10,500 feet (3,200 meters) in the ocean and stay down as long as an hour.

Michael J. Moore and Greg A. Early of the Woods Hole Oceanographic Institution found evidence of the bends in bones of modern sperm whales, but they also found the same damaged skeletons in whale bones up to 111 years old.

This suggests, said Moore, that sperm whales are neither anatomically or physiologically immune from the effects of deep diving, even though they spend much of their 70-year lifetime at great ocean depths.

What causes the bends?
Decompression illness is caused when an air breather, such as human or a whale, is put under great pressure, such as in a deep dive, followed by a quick release of the pressure, as happens when a diver surfaces too quickly.

Under great pressure, nitrogen inhaled from the atmosphere supersaturates the body's tissue. When the pressure is released suddenly, the nitrogen reverts to gas and forms bubbles in the tissue and in the blood. When the bubbles enter a vessel, they can block the flow of blood, starving the tissue of oxygen. When this happens in bone and cartilage, the bone dies and is not repaired, said Moore.

The result leaves pits and lesions in the bones. If there are repeated cases of bends, the injuries expand and eventually form deep gaps in the bone. In humans, this condition, called osteonecrosis, is typically caused by the bends.

Evidence in the bones
Moore and Early found the same condition when they examined the skeletons of sperm whales. They found that the older the animal was at death, the more bone damage from bends was evident.

Moore said the study shows that the decompression injury commonly experienced by the sperm whale "is not associated with any modern industrial or man-made changes over the last century."

Instead, it is a natural part of the life of the sperm whale.

"It is a cumulative, non-lethal cost of doing business for the sperm whale," said Moore.

He said sperm whales apparently avoid decompression injury by controlling how rapidly they surface to breathe and how long they spend on the surface.

As a result Moore said that any human activity that changes a whale's behavior could cause it to be further injured by the bends. For instance, said Moore, if acoustic signals from submarines or other human activities caused a sperm whale to surface too rapidly or to remain on the surface too long, it could trigger the bends and cause injury to the animal.

"If any acoustic stressors (such as submarine radio or sonar signals) were to override normal behavior, then they may run the risk of getting acute nitrogen problems which could cause pain and potentially strand them," said Moore. "This study opens the question that acoustic stressors may be impacting the normal physiology of these animals."

Speculation and study
Moore emphasized that any impact on sperm whales by manmade causes is only speculation.

However, a study last year found that some beaked whales that beached themselves in the Canary Islands after a military sonar test bore evidence of suffering from decompression illness, suggesting they were rapidly driven to the surface by noxious underwater sounds.

"This study is very important because it provides solid evidence to dispel the long-held belief that deep-diving whales are immune from the bends," said Phil Clapham, a whale expert with the National Oceanographic and Atmospheric Administration. "But beyond that, it's a significant piece of work because it shows the potential for whales to suffer serious consequences if they're forced to surface rapidly."

Clapham said there is a "growing body of evidence" that submarine sonar signals can have this effect on some deep-diving whales.

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